A photovoltaic device, such as a solar cell, is designed, in order to improve its performance, to capture solar light inside the device with high efficiency and to convert the captured light energy into electrical energy with high efficiency. One of the efforts to improve the performance is an effort in separating a PN junction. Most solar cells formed of a semiconductor material generate electrical power using photovoltaic power at a junction. Particularly, in a case of using crystalline silicon as the material, a PN junction is often formed by thermal diffusion.
As a method of forming a PN junction, thermal diffusion is known as a method that is excellent in terms of mass productivity and cost. However, in a thermal diffusion method, diffusion extends to the entire exposed potion of a substrate including the light-receiving surface (the front surface) and the back surface of the substrate. In the solar cell, it is necessary to form a PN junction on the light-receiving-surface side of the substrate, and a PN junction is not necessary on the back-surface side, which is the opposite side of the light-receiving surface, and the side surface of the substrate. If the PN junction is left on the light-receiving-surface side, the back-surface side, and the side surface, a short-circuit of a current is likely to occur between the light-receiving-surface side and the back-surface side, resulting in loss of the function as a solar cell.
For this reason, a process of manufacturing a photovoltaic device includes a process of separating or insulating an unnecessary junction between the light-receiving-surface side and the back-surface side. Specifically, a method such as, mainly, dry etching by plasma discharge, laser processing, and wet etching is often employed.
In the process of separating or insulating a junction, one of the points to be considered in order to improve the performance of the photovoltaic device and increase the efficiency is controllability in accurately leaving a junction in a necessary portion and removing a junction in a portion where a junction needs to be removed. Another point to be considered is to reduce damage on the substrate due to the processing as much as possible in order to reduce an adverse effect on the characteristics of the photovoltaic device.
While dry etching and laser processing do not cause many problems when they are applied to a standard solar cell, in order to increase the efficiency of the solar cell, dry etching and laser processing are unsuitable because they are inferior in controllability or reduction of damage. Wet etching is suitable for increasing the efficiency of the solar cell as a method of achieving both good controllability and reduction of damage.
Regarding junction separation by wet etching, for example, Patent Literature 1 proposes a technique of removing a junction on the back-surface side while leaving a junction on the light-receiving-surface side by precisely controlling a positional relationship of the fluid surface and the substrate in the height direction.